This invention relates to printing of bar codes on self-supporting substrates such as cartons or boxes using hot melt ink.
In order to detect information recorded in conventional bar codes accurately, each bar in the bar code should be completely black and the edges of each bar should be well defined. In low resolution ink jet printing systems using hot melt ink, however, ink drops are deposited on a substrate in spaced relation and freeze upon contact to produce a pixel pattern in which the ink dot representing each pixel is usually separated from adjacent dots so as to leave a space between them. At low resolution i.e., about 200 dots per inch (dpi) using black ink, and spaces between ink dots are normally detectable by the human eye and provide an overall gray appearance for areas intended to be solid black. As a result, automatic detection of the bars in a bar code may be erratic.
While liquid ink applied to a fiber substrate tends to spread and fill blank spaces between the locations of the drops, it also causes bleeding which produces raggedness of the edges of the bars in a bar code, interfering with the detection of spaces between the bars. Such bar code detection problems may be significantly reduced or eliminated by using high resolution ink jet printers, but high resolution ink jet printers are available only at substantially increased cost in comparison with low resolution ink jet printers.
Heretofore, solid coverage of hot melt ink images printed on a thin substrate such as paper has been improved by heating the platen on which the paper is supported during printing to cause the hot melt ink drops to flow and coalesce as described, for example, in U.S. Pat. Nos. 4,751,528 and 4,951,067, or by passing a paper substrate on which a hot melt ink image has previously been formed through a heating unit as described, for example, in U.S. Pat. Nos. 4,971,408 and 5,281,442. Such procedures, however, are not possible where a bar code is printed on a self-supporting surface such as a surface of a carton or box which cannot be placed on a heated platen or otherwise heated from behind the surface on which the bar code is printed.
Normally, the ink drops from a low resolution hot melt ink jet printer printing a bar code on a box or carton solidify substantially on impact with the carton, providing ink spots about 0.003 inch in diameter which are spaced by about 0.005 inch. While the spacing of the drops in the scanning direction can be reduced by moving the carton past the printer more slowly or by increasing the rate of drop ejection, such small drop sizes with low resolution ink jet printers leave substantial blank spaces between the drops on the surface of the carton in the cross direction i.e., the direction perpendicular to the scanning direction, which interferes with the accuracy of the bar code readings.